Use of xanthan gum as an anode binder

ABSTRACT

Xanthan gum has been found to be a superior binder for binding an electrode, especially an anode, in a lithium-ion or lithium-sulfur battery, being able to accommodate large volume changes and providing stable capacities in batteries tested with different types of anode materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/642,929 filed on Oct. 23, 2012, which is a National Phase Entry ofInternational PCT Patent Application Serial No. PCT/CA2011/000450 filedon Apr. 21, 2011 and which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/327,148 filed on Apr. 23, 2010, the entirecontents of which is herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to electrode binders, particularly anode bindersin lithium-ion or lithium-sulfur batteries.

BACKGROUND OF THE INVENTION

In lithium-ion batteries cathodes typically comprise alithium-containing material and the anode is usually a lithium-freematerial such as graphite, a metal, a metalloid, or an oxide. Inlithium-sulfur batteries, the anode is typically lithium metal and thecathode is made of sulfur or a carbon/sulfur composite. Newlithium-sulfur battery devices use a metal/carbon composite as an anodeand a polysulfide cathode (e.g. Li₂S). In lithium ion batteries, theanode may be: i) a metal or a metalloid that can alloy with lithium,mainly elements form groups: 2B (Zn and Cd), 3A (Al, Ga and In), 4A (Si,Sn and Pb), 5A (Sb and Bi) and Sn-alloys (Sn—Fe, Sn—Co, Sn—Ni, Sn—Cu,Sn—Zn); ii) hard or soft carbon (e.g. graphite), iii) an oxide whosemetal allows with lithium such as SnO₂, Sb₂O₃, and silicon oxide); iv) atransition metal oxide (Li₄Ti₅O₁₂, titanium oxide, chromium oxide,manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxideand zinc oxide). Lithium also intercalates into nitrides, phosphides andsulfides. In lithium-ion batteries, of particular importance are anodesin which lithium is intercalated into an oxide or an oxide-carbonmatrix.

Tin oxide reacts with lithium according to the following reaction:

SnO₂+4Li⁺+4e⁻→Sn+2Li₂O irreversible reaction (711 mAh/g)

Sn+4.4Li⁺+4.4e^(−⇄SnLi) _(4.4) reversible reaction (783 mAh/g)

Anodes that comprise metal (or a carbon/metal composite or metal oxide)that alloys with lithium suffer from large volume change uponlithiation/delithiation of the electrode during operation andre-charging of the battery. This volume expansion ranges from about 100%for Al to about 300% for Si. In order to accommodate this large volumechange it is necessary either to use nanoparticles or use a binder thatcan accommodate this volume change. Polyvinylidene fluoride (PVDF) isthe conventionally used binder in battery technology; however, it doesnot accommodate a volume change larger than about 15-20%, such as forgraphite or Li₄Ti₅O₁₂. PVDF does not get reduced at low potential (5 mVversus Li/Li⁺) nor oxidized at high potential (5 V versus Li/Li⁺) atroom temperature. However, at elevated temperature, it has been reportedthat PVDF reacts with Li metal and LiC₆ to form LiF and some C=CFspecies via an exothermic reaction which will cause a risk for thermalrunaway (Du Pasquier 1998; Maleki 1999; Maleki 2000). To avoid thisrisk, research has focused on the use of non-fluorinated binders(Gaberscek 2000; Oskam 1999; Ohta 2001; Zhang 2002; Verbrugge 2003).Even though they are still insoluble in water, reduced heat was obtainedwhen phenol-formaldehyde, poly(vinylchloride) or polyacrylonitrile areused as binders (Maleki 2000; Du Pasquier 1998). Another disadvantage ofusing PVDF is its price which is about US $20 per kg in North America(∈15-18 per kg in Europe (Lux 2010)). In addition, PVDF requires the useof non-environmentally friendly solvents to process the electrodeformation, such as N-methyl-2-pyrrolidone (NMP). Also, it is not easy todispose of PVDF at the end of the battery life (Lux 2003). Thus moreenvironmentally friendly binders are needed for preparing electrodematerials for Li-ion batteries.

Some rubber-based binders such as styrene-butadiene rubber have beentested with some success, but these binders are not water soluble andthere is a further need to improve their ability to accommodate volumeexpansion. Sodium carboxymethylcellulose (NaCMC) is a sugar-basedmolecule used as a thickener in the food industry and has shown goodaccommodation of volume expansion in the case of silicon-basedelectrodes (Li 2007; Buqa 2006; Beattie 2008; Hochgatterer 2008; Liu2005), and more recently with tin oxide-based electrodes (Chou 2010). Inaddition to being able to accommodate the volume change, NaCMC is watersoluble due to the carboxymethyl groups attached to the cellulose. Thisavoids the use of non-environmentally friendly solvents during thecasting process, which makes fabrication of the electrode easier. Asmentioned by Lux et al., the use of NaCMC also makes the recycling ofLi-ion battery anodes easier (Lux 2010). Indeed, by heating NaCMC at700° C., Na₂CO₃ is obtained. In addition (Machado 2003; Kaloustian1997), the price of NaCMC is much lower than PVDF, about US $6 per kg inNorth America (∈1-2 per kg in Europe (Lux 2010)).

One approach in the prior art (Satoh 2005; Satoh 2008; Satoh 2009a;Satoh 2009b) for producing negative electrodes for batteries has been touse graphite-based anodes and binders having olefinic unsaturated bonds(e.g. styrene-butadiene rubbers). In such an approach, thegraphite-based anode may be coated with metal oxides. This prior artalso suggests that xanthan gum may be used as a co-binder along with thebinder having olefinic unsaturated bonds. However, there has been nospecific exemplification of the use of xanthan gum and the use ofxanthan gum exclusively, i.e. not as a co-binder, has not beensuggested. Further, the anode material is restricted to graphite orgraphite coated on the surface with metal oxides. Such anode materialsdo not experience the very large volume expansions that oxide oroxide-carbon matrix materials undergo during the lithiation/delithiationprocess.

There remains a need in the art for water soluble binders which canaccommodate large volume expansions upon lithiation/delithiation ofelectrodes in lithium-ion or lithium-sulfur batteries.

SUMMARY OF THE INVENTION

It has now been surprisingly found that xanthan gum is an excellentwater-soluble binder for anodes in lithium-ion and lithium-sulfurbatteries, accommodating large volume changes and providing stablecapacities in batteries tested with different types of anode materials.

Thus, there is provided a use of a binder consisting essentially ofxanthan gum for binding an electrode in a lithium-ion or lithium-sulfurbattery.

There is further provided a use of xanthan gum as a binder for anelectrode consisting essentially of lithium intercalated into an oxideor lithium intercalated into a homogeneous matrix of an oxide and aconductive carbon.

There is further provided an anode for a lithium-ion or lithium-sulfurbattery comprising a lithium-containing material bound by a binderconsisting essentially of xanthan gum.

There is further provided an anode for a lithium-ion battery comprisinga xanthan gum-bound electrode comprising a lithium-containing materialconsisting essentially of lithium intercalated into an oxide or lithiumintercalated into a homogeneous matrix of oxide and a conductive carbon.

For general use in lithium-sulfur and lithium-ion batteries, the anodebinder consists essentially of xanthan gum, where no co-binder ispresent. In lithium-sulfur batteries, the lithium-containing material ofthe anode is Li—S. In lithium-ion batteries, anodes may comprise alithium-containing material, for example, a lithium alloy, lithiumintercalated into a conductive carbon (e.g. graphite, carbon black,mesoporous carbon microbeads, carbon nanotubes, graphene and mixturesthereof), lithium intercalated into an oxide (e.g. aluminum oxide, tinoxide, silicon oxide, cobalt oxide, iron oxide, titanium oxide, copperoxide and mixtures thereof), lithium intercalated into a nitride,lithium intercalated into a phosphide, or lithium intercalated intosilicon, or lithium inserted into a compound or composite bydisplacement. Lithium intercalated into a conductive carbon, lithiumintercalated into an oxide or lithium intercalated into a homogeneousmatrix of oxide and conductive carbon are preferred lithium-containingmaterials. Of the oxides, metal oxides are of particular note, moreparticularly transition metal oxides. Transition metal oxides include,for example, chromium oxides, manganese oxides, iron oxides, cobaltoxides, nickel oxides, copper oxides and zinc oxides. Tin oxide orsilicon oxide is preferred. Mixed metal oxides, for example ZnMn₂O₄ maybe used.

Of particular importance are anodes which consist essentially of lithiumintercalated into an oxide or an oxide-carbon matrix. The oxide-carbonmatrix is composite having a substantially homogeneous matrix of carbonin the oxide or an oxide in the carbon. Preferred oxides and conductivecarbons are described previously. The oxide:carbon ratio (w/w) ispreferably in a range of from 99:1 to 1:99, more preferably 90:10 to10:90, most preferably 89:11 to 11:89. The oxide is preferably providedin the form of nanoparticles, forming a nanocomposite with the carbon.In this particular application, the binder containing xanthan gum mayfurther comprise a co-binder, for example, polyvinylidene fluoride,sodium carboxymethylcellulose, styrene-butadiene rubber or mixturesthereof. Preferably, the co-binder is present in the binder in an amountof less than about 75 wt % of the weight of the binder, more preferablyless than about 50 wt %, even more preferably less than about 25 wt %.Most preferably, the binder does not contain a co-binder and consistsessentially of xanthan gum.

Also of importance are anodes comprising alloy composites incorporatingat least one element (metal or metalloid) that can alloy with lithium(e.g. Zn, Cd, Pt, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi), or a conductivecarbon as discussed earlier or a second element (preferably a metal)that does not alloy with lithium (e.g. Co or Ti). The use of xanthan gumhelps accommodate large volume expansion (100-400%) associated with thealloying process that leads to bad battery performance. Sn, Si andSn/Co/C alloy are of particular note.

Xanthan gum is a polysaccharide derived from the bacterial coat ofXanthomonas campestris. Xanthan gum is prepared by inoculatingXanthomonas campestris bacterium with a sterile aqueous solution ofcarbohydrate (e.g. glucose, sucrose, lactose or mixtures thereof), asource of nitrogen, dipotassium phosphate, and some trace elements. Themedium is well-aerated and stirred, and the polymer is producedextracellularly into the medium. The final concentration of xanthanproduced will vary greatly depending on the method of production, strainof bacteria, and random variation. After fermentation that can vary intime from one to four days, the xanthan polymer is precipitated from themedium by the addition of isopropyl alcohol and dried and milled to givea powder that is readily soluble in water or brine to form a gum. Thedegree of carboxylation of xanthan gum can affect its performance as anelectrode binder. Typically, a higher degree of carboxylation willconfer better battery performance. The degree of carboxyation of thexanthan gum may be readily controlled by carboxylation orde-carboxylation reactions to achieve the desired performancecharacteristics. Preferably, the degree of carboxylation is in a rangeof from about 0.5 to about 1.

Further features of the invention will be described or will becomeapparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, embodimentsthereof will now be described in detail by way of example, withreference to the accompanying drawings, in which:

FIG. 1 depicts a graph showing cycling behavior of half batteries madeof a lithium reference and counter electrode and a meso carbon microbead(MCMB) anode. Anodes were prepared using five different binders:polyvinylidene fluoride (PVDF), sodium carboxymethylcellulose (NaCMC)from two different sources, lithium carboxymethylcellulose (LiCMC),Baytron™ (a compound of poly-3,4-ethylenedioxythiophene andpolystyrenesulfonic acid) and xanthan gum. The electrodes were cycledbetween 5 mV and 1.5 V versus Li/Li⁺ at C/12 (complete charge anddischarge in 24 h).

FIG. 2 depicts a graph showing cycling behavior of half batteries madeof a lithium reference and counter electrode and a meso carbon microbead(MCMB) anode. Anodes were prepared using five different binders:polyvinylidene fluoride (PVDF), sodium carboxymethylcellulose (NaCMC),lithium carboxymethylcellulose (LiCMC), Baytron™ (a compound ofpoly-3,4-ethylenedioxythiophene and polystyrenesulfonic acid) andxanthan gum. The electrodes were cycled between 5 mV and 1.5 V versusLi/Li⁺ at different C-rates (C/12, C/9, C/6, C/3, C/2, C and 2C).

FIG. 3 depicts a graph showing a long term cycling behavior of halfbatteries made of a lithium reference and counter electrode and a mesocarbon microbead (MCMB) anode. Anodes were prepared using xanthan gum.The electrodes were cycled between 5 mV and 1.5 V versus Li/Li⁺ atdifferent C-rates.

FIG. 4 depicts a graph showing cycling behavior of half batteries madeof a lithium reference and counter electrode and an anode of a compositematerial (nano-SnO₂/C 78%-22% in weight %). Anodes were prepared usingthree different binders: polyvinylidene fluoride (PVDF), sodiumcarboxymethylcellulose (NaCMC), and xanthan gum. The electrodes werecycled between 0.1 V and 1 V versus Li/Li⁺ at C/12 (complete charge anddischarge in 24 h).

FIG. 5 depicts a graph showing cycling behavior of half batteries madeof a lithium reference and counter electrode and a Sn—Co anode. Anodeswere prepared using three different binders: polyvinylidene fluoride(PVDF), sodium carboxymethylcellulose (NaCMC), and xanthan gum. Theelectrodes were cycled between 5 mV and 1.5 V versus Li/Li⁺ at C/12.

FIG. 6 depicts a graph showing cycling behavior of half batteries madeof a lithium reference and counter electrode and an anode of a sinteredmixed metal oxide (ZnMn₂O₄ spinel). Anodes were prepared using fivedifferent binders: lithium carboxymethylcellulose (LiCMC), sodiumcarboxymethylcellulose (NaCMC), Baytron™ (a compound ofpoly-3,4-ethylenedioxythiophene and polystyrenesulfonic acid), xanthangum and polyvinylidene fluoride (PVDF). The electrodes were cycledbetween 10 mV and 3 V versus Li/Li⁺ at C/10 (complete charge anddischarge in 20 h).

FIG. 7 depicts a graph showing long term cycling behavior of halfbatteries made of a lithium reference and counter electrode and asilicon anode. Anodes were prepared using lithium carboxymethylcellulose(LiCMC), sodium carboxymethylcellulose (NaCMC), xanthan gum andpolyvinylidene fluoride (PVDF). The electrodes were cycled between 5 mVand 2.0 V versus Li/Li⁺.

DESCRIPTION OF PREFERRED EMBODIMENTS

Cyclic voltammetry and cell cycling were carried out on half cells using2325-type coin cells assembled in an argon-filled glove box. Cyclicvoltammograms were recorded using a BioLogic™ VMP3potentiostat/galvanostat. The potential of the working electrode wasswept at 0.1 mV s⁻¹ from open-circuit potential down to 5 mV (or 10 mV)versus Li/Li⁺, then swept up to 1.5 V (or 2 V or 3 V) versus Li/Li⁺;afterwards cells were cycled between 1.5 V (or 2 V or 3 V) and 5 mV (or10 mV) versus Li/Li⁺. Capacity measurements were performed bygalvanostatic experiments carried out on a multichannel Arbin batterycycler. The working electrode was first charged down to 5 mV (or 10 mV)versus Li/Li⁺ at different C-rates and then discharged up to 1.5 V (or 2V or 3 V) versus Li/Li⁺. The mass of active material used in thecalculation is the mass of the material used in the active electrode.

Working electrodes were prepared as follows. Active material (e.g.carbon graphite (MCMB), nano-SnO₂/C, Sn—Co, ZnMn₂O₄ or Si) was mixedwith 5 wt % of Super carbon (Timcal) and 5 or 10 wt % of binder.Electrode films were made by spreading the material onto a high puritycopper foil current collector (cleaned using a 2.5% HCl solution inorder to remove the copper oxide layer) using an automated doctor-bladeand then dried overnight at 85° C. in a convection oven. Individual diskelectrodes (0=12.5 mm) were punched out, dried at 80° C. under vacuumovernight and then pressed under a pressure of 0.5 metric ton. A lithiummetal disk (0=16.5 mm) was used as a negative electrode (counterelectrode and reference electrode). 70 μL of a solution of 1 M LiPF₆ inethylene carbonate/dimethyl carbonate (1:1, v/v) was used as electrolyteand spread over a double layer of microporous propylene separators(Celgard™ 2500, 30 μm thick, =2.1 mm). The cells were assembled in anargon-filled dry glove box at room temperature.

Referring to FIG. 1, it is evident that for half batteries comprisingmeso carbon microbead (MCMB) anodes, the use of xanthan gum as the anodebinder leads to significantly higher and more stable discharge capacityover 100 charging cycles than all of the use of any of the otherbinders, including PVDF. Referring to FIG. 2, it is evident that thisresult is consistent across a broad range of C-rates.

For half batteries comprising nano-SnO₂/C anodes, the use of xanthan gumas the anode binder leads to significantly higher and more stabledischarge capacity over tens of charging cycles than with the use ofPVDF, and the use of xanthan gum is comparable to the use of NaCMC (seeFIG. 4).

For half batteries comprising Sn—Co anodes, the use of xanthan gum asthe anode binder leads to significantly higher and more stable dischargecapacity over tens of charging cycles than with the use of PVDF, albeita somewhat lower discharge capacity than with the use of NaCMC (see FIG.5).

For half batteries comprising ZnMn₂O₄ spinel anodes, the use of xanthangum as the anode binder leads to higher and more stable dischargecapacity over tens of charging cycles than with the use of PVDF,although the use of xanthan gum led to somewhat lower discharge capacitythan the use of LiCMC or NaCMC (see FIG. 6).

For half batteries comprising silicon anodes, the use of xanthan gum asthe anode binder leads to higher discharge capacity retention at cycle#2 than PVDF (45% vs. 21%) but lower discharge capacity retention thaneither LiCMC (87%) or NaCMC (95%) (see FIG. 7).

Results demonstrate that the use of xanthan gum as an anode binderprovides higher and more stable discharge capacity than polyvinylidenefluoride over a broad range of anode materials, while in some casesbeing comparable to carboxymethylcellulose.

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Other advantages that are inherent to the structure are obvious to oneskilled in the art. The embodiments are described herein illustrativelyand are not meant to limit the scope of the invention as claimed.Variations of the foregoing embodiments will be evident to a person ofordinary skill and are intended by the inventor to be encompassed by thefollowing claims.

1. An electrode consisting essentially of lithium intercalated into anoxide or lithium intercalated into a homogeneous matrix of an oxide anda conductive carbon, the electrode bound by a binder comprising xanthangum.
 2. The electrode according to claim 1, wherein the electrodeconsisting essentially of lithium intercalated into an oxide.
 3. Theelectrode according to claim 1, wherein the electrode consistsessentially of lithium intercalated into a homogeneous matrix of anoxide and a conductive carbon.
 4. The electrode according to claim 3,wherein the conductive carbon comprises graphite.
 5. The electrodeaccording to claim 1, wherein the oxide is a metal oxide.
 6. Theelectrode according to claim 1, wherein the oxide is aluminum oxide, tinoxide, silicon oxide, cobalt oxide, iron oxide, titanium oxide, copperoxide or mixtures thereof.
 7. The electrode according to claim 1,wherein the oxide is tin oxide or silicon oxide.
 8. An anode for alithium-don or lithium-sulfur battery comprising a lithium-containingmaterial bound by a binder consisting essentially of xanthan gum.
 9. Theanode according to claim 8, wherein the lithium-containing materialconsists essentially of Li—S, a lithium alloy, lithium intercalated intoa conductive carbon, lithium intercalated into an oxide, lithiumintercalated into a nitride, lithium intercalated into a phosphide orlithium intercalated into silicon.
 10. The anode according to claim 8,wherein the lithium-containing material consists essentially of lithiumintercalated into an oxide.
 11. The anode according to claim 8, whereinthe lithium-containing material consists essentially of lithiumintercalated into a metal oxide.
 12. The anode according to claim 8,wherein the lithium-containing material consists essentially of lithiumintercalated into an aluminum oxide, tin oxide, silicon oxide, cobaltoxide, iron oxide, titanium oxide, copper oxide or mixtures thereof. 13.The anode according to claim 8, wherein the lithium-containing materialconsists essentially of lithium intercalated into tin oxide or siliconoxide.
 14. The anode according to claim 8, wherein thelithium-containing material consists essentially of lithium intercalatedinto a mixed metal oxide.